1. Field of the Invention
The invention relates to a temperature sensor for a spark plug of an internal combustion engine which is capable of measuring the temperature at the front end of a spark plug electrode.
2. Description of Prior Art
In a spark plug generally used for internal combustion engine, the temperature of the spark plug is measured for the purpose of appropriately arranging a degree of gas mixture, intake temperature, ignition timing, revolution speed, intake and exhaust pressure.
As one example of this kind of spark plug, the applicant of this application had previously suggested a model shown in Japanese Utility Model Publication No. 63-67835.
According to the Utility Model Publication No. 63-67835, the applicant have found the following disadvantages.
(i) No shield is taken into consideration, so that there is a tendency of leaking at any level of noise. The cord or cable which picks up the potential of the thermocouple, is electrically connected to a high tension line so that the cord or cable has tendency of inducing noise. Further, the electrically conductive case is connected to the high tension line so that noise is induced from the spark plug body, the plug cap, the case, the cord and the like. The noise influences measuring the temperature of the spark plug body.
(ii) The electrically conductive case is connected to the high tension cord so that the case is kept at high voltage to induce spark to the ground particularly when the grounding deteriorates.
(iii) To provide a shield with the case so as to avoid leakage, it is necessary to provide enough insulation, thus requiring a large device, and at the same time, making it cumbersome to handle.
(iv) The cable is fixebly secured to the converter so that it is necessary to individually provide a different connector each time a different spark plug is introduced.
(v) It is impossible to remove the converter from the case, and thus making it difficult to check the converter and replace battery cells.
Therefore, it is one object of this invention to obtain a temperature sensor which is capable of easily connecting to a spark plug body, and readily checking a converter and replacing battery cells.
It is another object of this invention to a obtain temperature sensor which is capable of reducing the level of noise, and at the same time, high voltage breakdown, and making easy to handle.
According to the present invention, the noise leaking from the outer insulator sleeve is shielded by the metallic sheet to contribute to the reduction of noise. The electrical connection among the high tension cord, the shield wire and the spark plug body, is facilitated by a single outer insulator sleeve.
The outer, middle and inner shaft is provided to connect to the triple coaxial plug, thus preventing the diameter of the spark plug body from the being unnecessarily enlarged.
Further, the high tension cord and the shield cord are placed into the outer insulator sleeve in addition to the shield coat being connected to the metallic sheet. This enables to protect the high tension cord and the shield wire against the leakage of noise.
The interconnecting case is enclosed by the shield case in electrically insulating relationship with the shield case. The shield case is avoided from being kept at high voltage to ensure safety. The interconnecting case is electrically insulated from the shield case so as to prevent electrical elements of the converter from being broken down.
Various other objects and advantages of the present invention will appear in the following description and in the accompanying drawings.